Embodiments of the invention relate generally to X-ray tubes and more particularly to a method and apparatus for beam focusing and control in an indirectly heated cathode.
Typically, in computed tomography (CT) imaging systems, an X-ray source emits a fan-shaped or cone-shaped beam toward a subject or an object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be used to include anything that is capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the X-ray beam by the subject. Each detector element of a detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis. The data processing system processes the electrical signals to facilitate generation of an image.
Generally, the X-ray source and the detector array are rotated about a gantry within an imaging plane and around the subject. Furthermore, the X-ray source generally includes an X-ray tube, which emits the X-ray beam at a focal point. Also, the X-ray detector or detector array typically includes a collimator for collimating X-ray beams received at the detector, a scintillator disposed adjacent to the collimator for converting X-rays to light energy, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Furthermore, currently available X-ray tubes typically include a filament that generates electrons. A cathode cup surrounds the filament to focus the electrons into an electron beam. The electron beam strikes an anode causing it to emit X-rays. Unfortunately, in these configurations, the filament has a limited life and low quality of emission especially for high power applications. Further, high power applications call for the filament to be heated to a high temperature, which results in evaporation of material of the filament. This evaporation of material in turn shortens the life of the filament. Also, in a filament emitter, due to the curved surfaces of coils, the electron beam leaving the emitter has some initial transverse velocity. This initial velocity lowers the beam quality and prevents the electron beam from forming a small size focal spot on the target.
Moreover, some currently available X-ray tubes employ indirectly heated cathodes. An indirectly heated cathode generally includes an emission source heated by an electron beam that is generated from a filament disposed behind the main emitter. This configuration unfortunately results in a non-uniform distribution of temperature at the emitter. It is therefore desirable to develop a design of an X-ray tube that has a long emitter life and enhanced beam quality.
Additionally, in order to facilitate a uniform temperature at the emitter, it is desirable to develop an indirectly heated cathode that has a capability to control the beam profile striking the emitter.